Processing poly(vinyl chloride) into useful objects requires a variety of adjuvants such as thermal stabilizers, processing aids, lubricants, optionally impact modifiers, pigments, and the like. In recent years, polymeric additives have been developed for many of these purposes, such as impact modification by core/shell polymers based on a butadiene or poly(alkyl acrylate) core surrounded by shells containing alkyl(meth)acrylate or vinyl aromatic polymers. Also, processing aids based on high molecular polymers of alkyl methacrylates are well-known. Such processing aids aid in the conversion of the PVC particles under conditions of heat and shear to a molten plastic capable of being shaped into the final desired object, the processing aids improving the fusion behavior and the processing of the melt. Further, special lubricating/processing aids based on polymers of alkyl (meth)acrylates have been developed which aid in fluxing and release from hot metal surfaces, further contributing relatively small increases to the melt viscosity of the PVC compound, which is important for certain processing operations, such as injection molding.
The producers of such polymeric additives have continued to seek for means of combining such additives beyond simple blending, so as to obtain an unexpected improvement in properties. One means has been to form core/shell polymers wherein, for example, a lubricating polymer is encapsulated by a processing aid polymer. Such encapsulation is best accomplished by sequential emulsion polymerization under conditions where the second polymer is prepared at conditions where no new particles are formed, and all the new polymer is formed at the pre-formed sites offered by the first polymer particles.
Simple blending of such core/shell emulsions prior to isolation, such as by coagulation or spray-drying, can create certain problems. Particles with low content of shell polymer may compact and not flow freely, or the lubricating component may deleteriously effect the fusion process, or dispersion of the additive polymers, either to their initial particle size in the PVC matrix or to miscible blends with the PVC, may be adversely affected.
Thus the compounder of poly(vinyl chloride) has sought a means for improving the efficacy of mixtures of polymeric additives, and the manufacturer has sought means to combine such more efficiently and to produce blends which handle well without compaction prior to admixture with the PVC.